Force -D__EXTENSIONS__ on Solaris.

This fixes compilation of libavformat/udp.o on some installations,
regression since 3a5cbc91
Reported by Cristian Dustinta for SunOS 5.10

configure

@@ -4040,6 +4040,7 @@ case $target_os in
         SHFLAGS='-shared -Wl,-h,$$(@F)'
         enabled x86 && SHFLAGS="-mimpure-text $SHFLAGS"
         network_extralibs="-lsocket -lnsl"
+        add_cppflags -D__EXTENSIONS__
         # When using suncc to build, the Solaris linker will mark
         # an executable with each instruction set encountered by
         # the Solaris assembler.  As our libraries contain their own